getSelectedItem() selects nothing

[alejandro garcia]

Hi! I'm trying to show the item selected in a combo box with getSelectedItem(), when the user clicks the button I just try to show the item with showMessageDialog, but it selects nothing and I can't find out why, could anybody help please?

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
 
public class Selector extends javax.swing.JFrame
{
/** Creates new form CAplicaci�n */
private javax.swing.JLabel jEtSaludo;
private javax.swing.JButton jBtSaludo;
  public Selector()      // constructor
  {
    setSize(300, 200);      // tama�o del formulario
    setTitle("Language Selector"); // t�tulo del formulario
    initComponents();       // iniciar controles o componentes
  }
     
    private void initComponents()//GEN-BEGIN:initComponents
  {
  
   getContentPane().setLayout(null);
      addWindowListener(new java.awt.event.WindowAdapter()
        {
          public void windowClosing(java.awt.event.WindowEvent evt)
            {
          exitForm(evt);
            }
        });
 
  
 String[] langStrings = { "Spanish", "French", "Dutch", "Chinese", "English" };
 
   JComboBox langList = new JComboBox(langStrings);
   langList.setSelectedIndex(4);
   getContentPane().add(langList);
langList.setBounds(42,90,204,30);

jbtAceptar= new javax.swing.JButton();
jbtAceptar.setText("Aceptar");
getContentPane().add(jbtAceptar);
jbtAceptar.setBounds(195,20,85,26);
   jbtAceptar.addActionListener(new java.awt.event.ActionListener()
    {
      public void actionPerformed(java.awt.event.ActionEvent evt)
      {
        jbtAceptarActionPerformed(evt);
      }
    });

  }//GEN-END:initComponents
   
 
private void jbtAceptarActionPerformed(java.awt.event.ActionEvent evt)
 {
  Object elementoSeleccionado;
  elementoSeleccionado = langList.getSelectedItem();
 javax.swing.JOptionPane.showMessageDialog(null,elementoSeleccionado);
 }
  
    
 
  /** Exit the Application */
  private void exitForm(java.awt.event.WindowEvent evt) {//GEN-FIRST:event_exitForm
    System.exit (0);
  }//GEN-LAST:event_exitForm

private javax.swing.JButton jbtAceptar;
private javax.swing.JComboBox langList;

  public static void main (String args[])
  {
    new Selector().setVisible(true);
  }
 

}

[Hui Zhao]

I guess showMessageDialog should display a string message rather than an object instance.

Object elementoSeleccionado;
  elementoSeleccionado = langList.getSelectedItem();
 javax.swing.JOptionPane.showMessageDialog(null,elementoSeleccionado);

???

[Freddy Wong]

I guess showMessageDialog should display a string message rather than an object instance.

No, that's not the problem. The problem is because you never initialize the langList.
Inside initComponents()

   
JComboBox langList = new JComboBox(langStrings);

You should remove the JComboBox since you already have the same variable name for the instance variable.

[alejandro garcia]

didn't work
Now I run this:
javax.swing.JOptionPane.showMessageDialog(null,"Hello");
and got the same error

[alejandro garcia]

I don't understand: I can't remove this line
JComboBox langList = new JComboBox(langStrings);
from the initComponents()

and I can't remove this attribute from the class:
private javax.swing.JComboBox langList;

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
 
public class Selector extends javax.swing.JFrame
{
/** Creates new form CAplicaci�n */
private javax.swing.JLabel jEtSaludo;
private javax.swing.JButton jBtSaludo;
private javax.swing.JComboBox langList;
 

  public Selector()      // constructor
  {
    setSize(300, 200);      // tama�o del formulario
    setTitle("Language Selector"); // t�tulo del formulario
    initComponents();       // iniciar controles o componentes
  }
     
    private void initComponents()//GEN-BEGIN:initComponents
  {
  
   getContentPane().setLayout(null);
      addWindowListener(new java.awt.event.WindowAdapter()
        {
          public void windowClosing(java.awt.event.WindowEvent evt)
            {
          exitForm(evt);
            }
        });
 
  
 String[] langStrings = { "Spanish", "French", "Dutch", "Chinese", "English" };
 
   JComboBox langList = new JComboBox(langStrings);
   langList.setSelectedIndex(4);
   getContentPane().add(langList);
langList.setBounds(42,90,204,30);

jbtAceptar= new javax.swing.JButton();
jbtAceptar.setText("Aceptar");
getContentPane().add(jbtAceptar);
jbtAceptar.setBounds(195,20,85,26);
   jbtAceptar.addActionListener(new java.awt.event.ActionListener()
    {
      public void actionPerformed(java.awt.event.ActionEvent evt)
      {
        jbtAceptarActionPerformed(evt);
      }
    });

  }//GEN-END:initComponents
   
 
private void jbtAceptarActionPerformed(java.awt.event.ActionEvent evt)
 {
 
  Object elementoSeleccionado;
  elementoSeleccionado = langList.getSelectedItem();
 javax.swing.JOptionPane.showMessageDialog(null,elementoSeleccionado);
 }
  
    
 
  /** Exit the Application */
  private void exitForm(java.awt.event.WindowEvent evt) {//GEN-FIRST:event_exitForm
    System.exit (0);
  }//GEN-LAST:event_exitForm
 

private javax.swing.JButton jbtAceptar;


  public static void main (String args[])
  {
    new Selector().setVisible(true);
  }
 

}

[Hui Zhao]

try this

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
 
public class Selector extends javax.swing.JFrame
{
/** Creates new form CAplicaci�n */
private javax.swing.JLabel jEtSaludo;
private javax.swing.JButton jBtSaludo;
private String[] langStrings = { "Spanish", "French", "Dutch", "Chinese", "English" };
JComboBox langList = new JComboBox(langStrings);
  public Selector()      // constructor
  {
    setSize(300, 200);      // tama�o del formulario
    setTitle("Language Selector"); // t�tulo del formulario
    initComponents();       // iniciar controles o componentes
  }
     
    private void initComponents()//GEN-BEGIN:initComponents
  {
  
   getContentPane().setLayout(null);
      addWindowListener(new java.awt.event.WindowAdapter()
        {
          public void windowClosing(java.awt.event.WindowEvent evt)
            {
          exitForm(evt);
            }
        });
 
  
 
 
   
   langList.setSelectedIndex(4);
   getContentPane().add(langList);
langList.setBounds(42,90,204,30);

jbtAceptar= new javax.swing.JButton();
jbtAceptar.setText("Aceptar");
getContentPane().add(jbtAceptar);
jbtAceptar.setBounds(195,20,85,26);
   jbtAceptar.addActionListener(new java.awt.event.ActionListener()
    {
      public void actionPerformed(java.awt.event.ActionEvent evt)
      {
        jbtAceptarActionPerformed(evt);
      }
    });

  }//GEN-END:initComponents
   
 
private void jbtAceptarActionPerformed(java.awt.event.ActionEvent evt)
 {
  Object elementoSeleccionado;
  elementoSeleccionado = langList.getSelectedItem();
 javax.swing.JOptionPane.showMessageDialog(null,elementoSeleccionado);
 }
  
    
 
  /** Exit the Application */
  private void exitForm(java.awt.event.WindowEvent evt) {//GEN-FIRST:event_exitForm
    System.exit (0);
  }//GEN-LAST:event_exitForm

private javax.swing.JButton jbtAceptar;
 

  public static void main (String args[])
  {
    new Selector().setVisible(true);
  }
 

}

[Freddy Wong]

I can't remove this line
JComboBox langList = new JComboBox(langStrings);

Is it because the IDE, i.e. NetBeans doesn't allow you to do that?

Actually what I was trying to say was that you never initialized the langList instance variable. To illustrate your problem, this might give you and idea why your code doesn't work.

    
public class MyClass
{
    private String str;
 
    public void init()
    {
        String str = "Hello World";
    }
 
    public String getString()
    {
        return str;
    }
 
    public static void main(String[] args)
    {
        MyClass m = new MyClass();
        // This will give you a NullPointerException
        // because actually you never initialize the str
        // instance variable. The str variable in the init() is
        // a local variable that appears to have the same name
        // as the instance variable.
        m.getString().toString();
    }
}

If you use NetBeans, when you drag a component into the JFrame, it normally creates an instance variable of that component and then in the initComponent() method, it will do the initialization.

Hope this helps.

[Michael Dunn]

> I don't understand: I can't remove this line
> JComboBox langList = new JComboBox(langStrings);
> from the initComponents()

> and I can't remove this attribute from the class:
> private javax.swing.JComboBox langList;

you have duplicate declarations for 'langList'

by including the 'type' JComboBox you make langList local to initComponents()
the other langList is not initialised, and is the one referenced from the button

String[] langStrings = { "Spanish", "French", "Dutch", "Chinese", "English" };
JComboBox langList = new JComboBox(langStrings);//<---
langList.setSelectedIndex(4);
should be
String[] langStrings = { "Spanish", "French", "Dutch", "Chinese", "English" };
langList = new JComboBox(langStrings);//<---
langList.setSelectedIndex(4);